1. Field of the Invention
The present invention relates to devices and processes for non-destructively scanning the interior of solid materials with sonic to ultrasonic frequency stress waves. More particularly, the present invention relates to devices and methods for surface monitoring of acoustic vibrations transmitted at least partially through a solid material specimen for interpreting the internal characteristics of that specimen. Further, the present invention relates to devices and methods to provide cost effective, non-destructive scanning and testing of the internal integrity of solid materials including concrete, wood, masonry, stone, steel, etc., with regard to flaws such as cracking, delamination, honeycomb, deterioration and the like. The present invention is a sonic/ultrasonic scanner with compression (P), shear (S), and surface (R or Rayleigh) wave measurement capabilities especially suited for assisting with the sensing and mapping of the internal characteristics of solid structural materials such as structures fabricated of concrete. The present invention has particular utility for non-destructively determining the internal characteristics and quality of structures in place and in their normal environment.
2. Description of the Prior Art
Contemporary non-destructive testing and evaluation of solid structures sometimes employ sonic and supersonic signals introduced into the test specimen to reflect the condition of its interior. Internal defects of the specimen cause slower velocity and lower amplitude signals while severe defects can block the signal entirely. Changes in specimen density and stiffness create acoustic impedances such as is encountered at the boundary wall of the specimen. Reflector depths are calculated based on the echo return time or the resonant frequency of the echo and the concrete wave velocity. Approximate depths of cracks or breaks, voids, soil intrusions, poor quality concrete, honeycomb consolidation problems, and enlargements are determinable by seismic echo and impulse response techniques.
Test data is sometimes obtained by drilling or embedding bore holes parallel to the structure in question, filling those holes with water, and introducing one or more hydrophones into the bore. Wave velocity is typically estimated from ultrasonic pulse velocity measurements sometimes obtained by one or more geophones embedded in the structure.
For direct, through transmission measurement of P and S wave travel in materials with contemporary devices and methods, it is necessary when testing a concrete slab, wall, column or other member, to manually place, on a repetitive basis, both a transmitting and receiving transducer at specific grid locations. An ultrasonic pulse is then introduced via a transmitting transducer, such as a piezoelectric crystal, which converts an electrical pulse into mechanical energy or, in some instances, by striking the surface under test with a small hammer of specific weight. The hammer might have a force transducer associated therewith to measure the force imposed and to signal the occurrence of the impact.
The P and/or S wave produced by either of the above methods travels through the concrete and is sensed by a receiving transducer which might typically also be a piezoelectric crystal. The receiver converts the mechanical wave energy into a corresponding electrical signal. This signal is then collected by an instrument that measures the travel time of the ultrasonic pulses between the source and receiver. An oscilloscope is often used to measure the travel time and record the receiver signal voltage. This procedure for ultrasonic pulse velocity (UPV) measurement is specified for concrete as ASTM C597-83. One of the more significant drawbacks to this method is the time required to manually place, operate, and then relocate the transducers so as to cover an adequate number of points of a grid.
It is known to ultrasonically test slab materials by employing a transducer contained in a roller which is manually movable over the test specimen. U.S. Pat. Nos. 3,628,375 by Pagano and 3,732,444 by Miller are examples. Miller configures the sensing piezoelectric crystal as a cylinder with a protective coating thereover. However, it suffers a serious disadvantage of coupling force variations between the roller and specimen as a direct result of its dependence on manual manipulation to engage the specimen surface. It also provides no indication of the amount of travel over the specimen surface.
Ultrasonic material testing at a fixed station is shown in U.S. Pat. No. 3,423,991 by Collins. It discloses a stationary type ultrasonic testing system for sheets of plywood at a station which uses a pair of cylindrical piezoelectric crystals within rollers. The functionally interchangeable roller transducers are mounted in a fixed frame with spring biasing towards one another. Sheets of plywood are passed between this pair of rollers with one of the fixed rollers actuated as a transmitter and the other operated as a sensor. Collins also discusses adjustability of the angle of incidence of the ultrasonic beam relative to the test specimen as well as operation of a transducer in an echo mode via pulse modulation of the transducer while it is in contact with the specimen.
It is known to interface data processing equipment including displays and computers with ultrasonic testing devices to identify probe locations, produce quality pattern displays, and so forth. Examples are shown in U.S. Pat. Nos. 4,160,386 by Jackson et al, 4,457,176 by Scholz, 4,594,895 by Fujii, 4,599,899 by Jero, 4,646,748 by Fukii, and 4,916,535 by Volodchenko et al. Jackson et al suggest using a multiplicity of sensors in fixed array for making it possible to locate a hand-held inspection probe by detecting signals arriving at the multiplicity of sensors.
Yet another contemporary ultrasonic testing procedure employs a separate hammer mechanism at a first, relatively fixed, location to impact the test specimen while sensing the arrival of the waves therefrom with a sensing transducer at another relatively fixed location. Such devices frequently include a force transducer for providing a feedback signal to the data processing equipment marking the occurrence of a test impact on the specimen by the hammer.
Impact Echo (IE) testing is also a known procedure. An Impact Echo test involves the introduction of an impact and the subsequent monitoring of the surface vibration response in time to identify resonant echoes indicative of the condition of the test member. The IE test involves measurement of the receiver time domain response and subsequent Fast Fourier Transform (FFT) analyses to provide linear displacement spectra of the natural resonant frequency response in structural members to determine their geometry and the presence of flaws in concrete and other solid materials from only one side of the test surface.
It is understood that the IE method was developed for use with P-wave energy at the National Institute of Standards and Technology by Dr. Nicholas Carino and Dr. Mary Sansalone in the 1980's for evaluation of slabs and other structural concrete members. It is further understood the development was continued by Dr. Sansalone at Cornell University for a point-specific, non-moving IE test system.
A significant disadvantage of the prior art IE systems is the time and inconvenience in setting up the sensors so as to obtain reliable data. That is, none of the known prior art devices are suitable for rapid and reliable production of substantial test data from specimens while in its normal environment such as concrete walls and columns of a structure. These and other disadvantages are overcome by the present invention as is described in greater detail below.
It is believed Spectral-Analysis-of-Surface-Wave (SASW) measurements were developed by Dr. Kenneth H. Stokoe, II. The current state of the art in SASW measurement involves coupling two transducers to the test medium at a given spacing and then impacting one, or both, ends of the receiver line at approximately the same distance away from the closest receiver as the receiver spacing. The SASW test involves measuring the surface wave propagation velocity between the receivers as a function of wavelength.